1. Field
The present invention relates to a plasma generating apparatus and a plasma generating method.
2. Description of the Related Art
Recently, the demand for air quality controls in living environments, such as sterilization and deodorization, is increasing due to an increase in symptoms like atopy, asthma, and allergies and an increase in the risk of infections such as new influenza in the population. Furthermore, as living conditions become more and more affluent, the amount of stored food or chances of storing uneaten foods increases, and thus it has become more and more important to control environments in food storage devices, such as refrigerators.
Related arts for controlling air quality in living environments are generally related to physical controls, such as filters. Relatively large dusts and particles floating in the air may be trapped by using physical controls. Depending on the size of filter holes, germs or viruses may also be trapped by using physical controls. Furthermore, in a case of physical control unit having innumerable absorption sites, such as activated carbon, even malodor molecules may be trapped. However, to trap such malodor molecules, it is necessary to transmit all the air in a space to be controlled through a filter, thus resulting in an increase in the size of a device and maintenance costs for filter replacements. Furthermore, such physical control is ineffective against malodor molecules attached to something. Therefore, an example of means for sterilizing or deodorizing malodor molecules attached to something is to release chemically active species into a space to be sterilized or deodorized. For spraying chemicals, air fresheners, or deodorizers, it is necessary to prepare the chemically active species in advance, and thus it is inevitable to periodically restock such chemically active species. Recently, methods for generating plasma in the air and sterilizing or deodorizing by using chemically active species generated therefrom are becoming popular.
Methods for generating plasma in the air by using electric discharge and sterilizing or deodorizing by using ions or radicals (referred to hereinafter as “chemically active species”) generated therefrom may be categorized into two types:
(1) So-called passive plasma generating apparatuses which make germs or viruses floating in the air (referred to hereinafter as “floating germs”) or malodorous substances (referred to hereinafter as “malodors”) react with active species within a space with limited volume within the passive plasma generating apparatuses (e.g., Patent Reference 1).
(2) So-called active plasma generating apparatuses which spray active species generated by a plasma generating unit into a closed space with a volume larger than that in (1) above (e.g., living room, bathroom, interior of a vehicle, etc.), such that the active species in the art collide and react with floating germs or malodors in the art (e.g., Patent Reference 2).
Since a passive plasma generating apparatus of (1) generates plasma within a relatively small volume, active species are densely generated and thus highly effective sterilization and deodorization may be expected. However, since it is necessary to introduce floating germs or malodors into the passive plasma generating apparatus, the size of the plasma generating apparatus is relatively large. Furthermore, ozone may be easily generated as a by-product of the plasma generation, and thus, it is necessary to additionally install a filter for absorbing or decomposing ozone to prevent ozone from leaking out of the plasma generating apparatus.
On the other hand, an active plasma generating apparatus of (2) may be manufactured to have a relatively small size, and not only sterilization of floating germs and decomposition of malodors in the art, but also sterilization of germs attached to surfaces of clothing or household items (referred to hereinafter as “attached germs”) and decomposition of malodors attached to surfaces of clothing or household items may be expected. However, since active species spread into a closed space that is excessively large compared to the volume of the active surfaces of clothing or household items, the concentration of the active species decreases, and thus, a sterilization or deodorization effect may only be expected with active species having a relatively long lifespan. Therefore, little deodorization effect may be expected in a space with a high concentration of malodors (concentration that is about 10,000 times the concentration of active species).
As described above, a passive plasma generating apparatus is only effective against floating germs or malodors contained in the air flowing into the passive plasma generating apparatus, whereas an active plasma generating apparatus is practically only effective against floating germs, attached germs, and malodors with relatively low concentrations. In other words, a function of the related art is restricted only one of “sterilization and deodorization of floating germs” or “sterilization of floating germs and attached germs with relatively low concentrations and deodorization of floating and attached malodors with relatively low concentrations”.
Furthermore, electrodes constituting a plasma generating unit commonly employ porous dielectric layers, for example, at portions of the electrodes at which plasma is generated. Therefore, under conditions of high humidity, moisture absorption of a dielectric layer changes the electric properties of the dielectric layer, and thus the generation of plasma is diminished. Particularly, in an environment with a low temperature and changeable humidity, such as a refrigerator, dew may easily condense on the dielectric layers of the electrodes. As a result, plasma generation is stopped and the efficiencies of sterilization and deodorization deteriorate. Therefore, if high humidity is maintained in a refrigerator, it is difficult to maintain the efficiency of sterilization.